Besides high-power light-emitting diodes (LEDs) with dimensions in the range of mm, micro-LEDs (μLEDs) are increasingly gaining interest today, motivated by the future applications of μLEDs in augmented reality displays or for nanometrology and sensor technology. A key aspect of this miniaturization is the influence of the structure size on the electrical and optical properties of μLEDs. Thus, in this article, investigations of the size dependence of the electro-optical properties of μLEDs, with diameters in the range of 20 to 0.65 μm, by current–voltage and electroluminescence measurements are described. The measurements indicated that with decreasing size leakage currents in the forward direction decrease. To take advantage of these benefits, the surface has to be treated properly, as otherwise sidewall damages induced by dry etching will impair the optical properties. A possible countermeasure is surface treatment with a potassium hydroxide based solution that can reduce such defects.

InGaN micro-LEDs (μLEDs) with their potential high-volume applications have attracted substantial research interest in the past years. In comparison to other III–V semiconductors, InGaN exhibits a reduced susceptibility toward non-radiative surface recombination. However, efficiency degradation becomes more prominent as dimensions shrink to a few μm or less. Due to the high surface-to-volume ratio of the miniaturized devices, the non-radiative recombination increases and reduces the internal quantum efficiency. While many groups focus on surface passivation to mitigate surface defects, the influence of crystallographic orientation of the μLED sidewall on the efficiency remains unexplored. This study addresses this gap by investigating the impact of crystallographic orientation of the sidewalls on the emission properties of the μLEDs. Hexagonal and elongated μLEDs with dimensions as small as 3.5 μm and sidewalls with crystallographically well-defined m- and a-planes were fabricated. Electrical and optical properties were investigated using photo- and electroluminescence. External quantum efficiency (EQE) is assessed based on well-known carrier recombination models. It can be shown that μLED performance intrinsically depends on the crystallographic orientation of the sidewalls. Comparing hexagonal μLED structures with a-plane and m-plane sidewalls, an increase in the EQE by 33% was observed for structures with a-plane sidewalls, accompanied by reduction in the current density of the peak EQE by a nearly two orders of magnitude compared to structures with m-plane sidewalls. By analyzing the EQE characteristics at the μLED center and near the sidewalls, the improvements can be directly attributed to the increased radiative recombination from sidewalls with a-plane orientation.

MicroLED arrays with the capability of switching each pixel separately with high frequency can serve as structured micro-illumination light engines for applications in sensing, optogenetics, microscopy and many others. We describe a scalable chip process chain for the fabrication of passive-matrix microLED arrays, which were integrated with PCB-based driving electronics. The arrays were produced by deep-etching of conventional planar LED structures on sapphire, followed by filling and planarization steps. The pixel resolution lies in the range of 254 to 2540 pixels-per-inch (ppi), the arrays consist of 32 x 32 pixels. Optical output powers up to 50 µW per pixel were measured. In comparison to CMOS-based approaches, the presented technology is a simplified strategy to produce microLED arrays with high pixel counts.

As micro-LEDs are further miniaturized for applications in high-resolution displays, size is becoming increasingly important for overall efficiency. To achieve high internal quantum efficiency, non-radiative recombination processes at the sidewall must be well understood and adequately mitigated. For this purpose, blue InGaN/GaN micro-LEDs were defined under two different dry etching conditions by changing the plasma power and Ar/Cl2 ratio, resulting in LEDs that were either stronger physically etched or stronger chemically etched. The LEDs were investigated using monochromatic cathodoluminescence imaging and time-resolved cathodoluminescence to determine how the etching conditions affect the detrimental influence of the sidewall on the quantum efficiency. A significant reduction of cathodoluminescence intensity and lifetime of the stronger physically etched structure from the bulk material toward the sidewall is observed, which is caused by an increase in the non-radiative recombination activity. Conversely, the stronger chemical etching conditions do not lead to a considerable degradation of the luminescence properties near the sidewall, which is attributed to the passivation of near-surface point defects with hydrogen during the etching process. This is corroborated by the intentional depassivation of such defects by low-energy electron beam irradiation or thermal annealing in an inert gas atmosphere leading to a similar efficiency deterioration, as well as by the recovery of efficiency by hydrogen plasma treatment. Overall, the results demonstrate that hydrogen can very effectively restore the efficiency of micro-LEDs at the sidewall. Therefore, the possible influence of hydrogen on the point defects at the sidewall should be considered in future studies investigating sidewall treatments.

Micro-LEDs generally suffer a loss of efficiency when they are reduced in size, as the sidewall has an increasingly greater impact on overall performance and promotes non-radiative surface recombination. However, as small micro-LEDs are in high demand due to promising applications in high-resolution displays, the influence of the sidewall must be well understood and mitigated by improvements in fabrication, e.g., directly during the etching process or by surface treatment. To improve the sidewall quality, blue InGaN/GaN micro-LEDs were defined under two different dry etching conditions by changing the plasma power and Ar/Cl2 ratio, and their sidewalls were treated either by plasma-enhanced chemical vapor deposition of SiOx or KOH wet chemical etching. The influence of the different surface conditions is analyzed by cathodoluminescence experiments to investigate how the distance between the excitation area and the sidewall affects the luminescence behavior. The distance dependence is significantly different for the investigated etching conditions, which is attributed to different degrees of surface passivation by hydrogen incorporated during dry etching. In addition, strong differences are observed for excitation close to or far from the sidewall, which cannot be explained only by differences in surface recombination, but can be ascribed to lateral confinement of charge carriers by the sidewall restricting spreading by diffusion. Lastly, the results strongly suggest that KOH etching not only removes the surface damaged by the etching process, but also improves the sidewall quality most likely due to the interaction of the etching by-product NH3 with GaN.

Herein, carbon‐implanted high‐temperature annealed (HTA) AlN layers are analyzed and donor–acceptor pair (DAP) transitions probably between the two most abundant impurities, carbon and oxygen, are identified. Both are regarded as the main, hard‐to‐avoid impurities in crystal growth. Oxygen is believed to lead to absorption in the deep UV below a wavelength of 250 nm. In contrast, carbon is the most likely candidate to be responsible for a distinct absorption band around 265 nm. This interpretation has recently been challenged. In this study, carbon‐implanted and HTA AlN layers with ion fluences above 8.1 × 10¹⁵ cm⁻² are analyzed using low‐temperature and time‐resolved cathodoluminescence spectroscopy. Due to the high concentration of oxygen inside the AlN, as a result of the HTA process, a DAP transition between a most likely carbon‐related acceptor and O_N is observed. The measured temperature‐ and power‐dependent blueshift of the peak emission energy as well as the luminescence transients can be clearly explained by a continuous change from a DAP transition at low temperature to a free electron to acceptor transition with increasing temperature. The findings are supported by a configurational coordinate model that describes the measured behavior qualitatively.